Conventionally, most of imaging devices of digital cameras and digital VTR cameras use CCD and MOS solid image-sensing devices. The optical system of such an image-sensing device requires a filter optical element capable of cutting near infrared light. In solid image-sensing devices, generally, the spectral sensitivity, for example, of CCD extends from a visible light region to the vicinity of 1,000 nm in a near infrared light region, so that it is required to cut near infrared light for matching the spectral sensitivity received through CCD to the counterpart of human eyes. Otherwise, an image obtained comes to be reddish, and no excellent image reproduction can be attained. For this purpose, filter glasses for cutting near infrared light have been developed and widely employed. For example, JP-A-4-104918 discloses a phosphate glass and a fluorophosphate glass.
Meanwhile, as imaging devices are downsized in recent years, it is strongly demanded to downsize image-sensing optical systems. For coping with the downsizing, it is sufficient to decrease the number of parts by using an optical element having a plurality of optical functions. For example, a thinkable solution is to impart one optical element with the above near infrared light cutting function and the function of an optical low-pass filter that passes light having a low spatial modulation frequency, or to impart one optical element with the near infrared light cutting function and a lens function.
Concerning lens functions as well, when the optical element is an aspherical lens, excellent optical performances can be obtained while using fewer parts as compared with an optical system using a spherical lens alone.
However, the above low-pass filter and the above aspherical lens have a problem that they cannot be produced highly productively when an infrared-light-absorbing glass is mechanically processed. For overcoming this problem, a near-infrared-absorbing glass can be precision-press molded to produce an optical element having the above multi-functions.
Since, however, a conventional near-infrared-absorbing glass contains arsenic, a large amount of P2O5 or fluorine for attaining a high transmittance at and around 400 nm, such a glass is not suitable as a glass for precision press-molding.
Arsenic involves environmental problems due to its toxicity. Further, it exhibits very high oxidizability, so that it may damage the molding surface of a press mold used for the precision press-molding and may make it impossible to use the mold repeatedly.
Further, a glass containing a large amount of P2O5 has a problem that such a glass is poor in climate resistance. When precision press-molding preforms are produced from such a glass and stored, the surface state of the preforms is deteriorated, and the preforms are no longer suitable as materials for producing optical elements by precision press-molding.
The introduction of fluorine gives a glass having a lower melting point and having an excellent transmittance. However, this glass has the following problem. Since fluorine volatilizes when the glass is melted, it is difficult to stably produce preforms one by one from the glass in a molten state, and such a glass is not suitable for the precision press-molding. Therefore, no optical element has been put to practical use that is produced from a glass described in the above JP-A-4-104981.